This invention relates generally to frequency converters for supplying multiphase loads, and more particularly, to a frequency converter for supplying a multiphase load, the frequency converter having a predetermined number of subconverters with AC voltage outputs, one output of each subconverter being brought to a common converter neutral point which is not connected to the multiphase load. Output current is impressed upon each subconverter by a separate current regulator.
Converters are frequently used to supply electrical energy to multiphase loads, having illustratively m phases. Such converter arrangements consist of m subconverters, where each subconverter has two AC outputs, one such output being brought to a common neutral point of the converter. A separate current regulator is advantageously utilized to impress the output current upon each subconverter during current-controlled operation of the converter. In situations where the neutral point of the converter is connected to a neutral point of a load, the current regulators will determine the magnitude of the currents flowing through each converter output.
U.S. Pat. No. 4,008,428 describes an arrangement wherein the neutral point of the converter is not connected to the neutral point of the machine because the machine current has a lower harmonic content, and the converter can be operated with a trapezoidal output voltage. Such a trapezoidal output voltage results in improved efficiency of the converter and the machine. However, since the sum of the currents at the neutral points of the converter must, in accordance with Kirchhoff's Law, always equal zero, the current regulators overdetermine the currents. If current regulators of the type having an integral component are used, the output voltages of the current regulators can diverge from one another, thereby electrically shifting the neutral point of the converter so that it is no longer at the center of the output phases. This produces an asymmetrical system. As mentioned in the above-identified reference, a common, additional signal is added to the reference values of all m current regulators as a supplemental reference value which is derived from the voltage difference between the converter neutral point and the load neutral point, and this voltage difference is regulated so as to equal zero. This arrangement, however, requires a voltage measuring device at each of the converter and machine neutral points. Such a system is costly, particularly in high-voltage installations which operate at greater than 1,000 volts. In addition, if a load neutral point is inaccessible, an artifical neutral point must be created by means of load resistors.
It is, therefore, an object of this invention to provide a simple and relatively inexpensive multiphase converter arrangement.